FIELD OF THE INVENTION
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The present invention relates to the field of hydrocarbon resource recovery, and, more particularly, to hydrocarbon resource recovery using RF heating.
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OF THE INVENTION
Energy consumption worldwide is generally increasing, and conventional hydrocarbon resources are being consumed. In an attempt to meet demand, the exploitation of unconventional resources may be desired. For example, highly viscous hydrocarbon resources, such as heavy oils, may be trapped in tar sands where their viscous nature does not permit conventional oil well production. Estimates are that trillions of barrels of oil reserves may be found in such tar sand formations.
In some instances these tar sand deposits are currently extracted via open-pit mining. Another approach for in situ extraction for deeper deposits is known as Steam-Assisted Gravity Drainage (SAGD). The heavy oil is immobile at reservoir temperatures and therefore the oil is typically heated to reduce its viscosity and mobilize the oil flow. In SAGD, pairs of injector and producer wells are formed to be laterally extending in the ground. Each pair of injector/producer wells includes a lower producer well and an upper injector well. The injector/production wells are typically located in the pay zone of the subterranean formation between an underburden layer and an overburden layer.
The upper injector well is used to typically inject steam, and the lower producer well collects the heated crude oil or bitumen that flows out of the formation, along with any water from the condensation of injected steam. The injected steam forms a steam chamber that expands vertically and horizontally in the formation. The heat from the steam reduces the viscosity of the heavy crude oil or bitumen which allows it to flow down into the lower producer well where it is collected and recovered. The steam and gases rise due to their lower density so that steam is not produced at the lower producer well and steam trap control is used to the same affect. Gases, such as methane, carbon dioxide, and hydrogen sulfide, for example, may tend to rise in the steam chamber and fill the void space left by the oil defining an insulating layer above the steam. Oil and water flow is by gravity driven drainage, into the lower producer well.
Operating the injection and production wells at approximately reservoir pressure may address the instability problems that adversely affect high-pressure steam processes. SAGD may produce a smooth, even production that can be as high as 70% to 80% of the original oil in place (OOIP) in suitable reservoirs. The SAGD process may be relatively sensitive to shale streaks and other vertical barriers since, as the rock is heated, differential thermal expansion causes fractures in it, allowing steam and fluids to flow through. SAGD may be twice as efficient as the older cyclic steam stimulation (CSS) process.
Many countries in the world have large deposits of oil sands, including the United States, Russia, and various countries in the Middle East. Oil sands may represent as much as two-thirds of the world's total petroleum resource, with at least 1.7 trillion barrels in the Canadian Athabasca Oil Sands, for example. At the present time, only Canada has a large-scale commercial oil sands industry, though a small amount of oil from oil sands is also produced in Venezuela. Because of increasing oil sands production, Canada has become the largest single supplier of oil and products to the United States. Oil sands now are the source of almost half of Canada's oil production, although due to the 2008 economic downturn work on new projects has been deferred, while Venezuelan production has been declining in recent years. Oil is not yet produced from oil sands on a significant level in other countries.
U.S. Published Patent Application No. 2010/0078163 to Banerjee et al. discloses a hydrocarbon recovery process whereby three wells are provided, namely an uppermost well used to inject water, a middle well used to introduce microwaves into the reservoir, and a lowermost well for production. A microwave generator generates microwaves which are directed into a zone above the middle well through a series of waveguides. The frequency of the microwaves is at a frequency substantially equivalent to the resonant frequency of the water so that the water is heated.
Along these lines, U.S. Published Application No. 2010/0294489 to Dreher, Jr. et al. discloses using microwaves to provide heating. An activator is injected below the surface and is heated by the microwaves, and the activator then heats the heavy oil in the production well. U.S. Published Application No. 2010/0294489 to Wheeler et al. discloses a similar approach.
U.S. Pat. No. 7,441,597 to Kasevich discloses using a radio frequency generator to apply RF energy to a horizontal portion of an RF well positioned above a horizontal portion of an oil/gas producing well. The viscosity of the oil is reduced as a result of the RF energy, which causes the oil to drain due to gravity. The oil is recovered through the oil/gas producing well.
Unfortunately, long production times, for example, due to a failed start-up, to extract oil using SAGD may lead to significant heat loss to the adjacent soil, excessive consumption of steam, and a high cost for recovery. Significant water resources are also typically used to recover oil using SAGD, which impacts the environment. Limited water resources may also limit oil recovery. SAGD is also not an available process in permafrost regions, for example.
Moreover, despite the existence of systems that utilize RF energy to provide heating, such systems may suffer from inefficiencies as a result of impedance mismatches between the RF source, transmission line, and/or antenna. These mismatches become particularly acute with increased heating of the subterranean formation. Moreover, such applications may require high power levels that result in relatively high transmission line temperatures that may result in transmission failures. This may also cause problems with thermal expansion as different materials may expand differently, which may render it difficult to maintain electrical and fluidic interconnections.
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OF THE INVENTION
It is therefore an object of the invention to provide enhanced operating characteristics with RF heating for hydrocarbon resource recovery systems and related methods.
These and other objects, features, and advantages are provided by an apparatus for heating a hydrocarbon resource in a subterranean formation having a wellbore extending therein. The apparatus includes a radio frequency (RF) antenna configured to be positioned within the wellbore, a transmission line configured to be positioned in the wellbore and coupled to the RE antenna, and an RF source configured to be coupled to the transmission line. The apparatus also includes a balun configured to be coupled to the transmission line adjacent the RF antenna within the wellbore. The balun comprises a body defining a liquid chamber configured to receive a quantity of dielectric liquid therein. Accordingly, the balun may advantageously reduce common mode currents in an outer conductor of the RF transmission line, for example, as the operating characteristics of the RF antenna change during the heating process to thereby provide enhanced efficiencies.
More particularly, the balun may comprise an adjustable balun configured to receive an adjustable quantity of dielectric liquid therein. The transmission line may comprise a coaxial transmission line, and the body may comprise a tubular body surrounding the coaxial transmission line. More particularly, the tubular body may include an electrically conductive portion, and an insulating portion longitudinally between the electrically conductive portion and the RF antenna. Furthermore, at least one shorting conductor may be electrically coupled between the electrically conductive portion and the coaxial transmission line.
In addition, the coaxial transmission line may have a liquid passageway therein, and the balun may further comprise at least one valve for selectively communicating dielectric liquid to the liquid chamber. Also, the at least one valve may comprise a pressure-actuated valve, and the apparatus may further include a pressure source coupled in fluid communication with the liquid dielectric. The coaxial transmission line may include a tubular inner conductor and a tubular outer conductor surrounding the tubular inner conductor, and the tubular body may be coaxial with the tubular inner conductor and the tubular outer conductor. The apparatus may further include a liquid-blocking plug positioned adjacent an end of the liquid chamber. The apparatus may also include a liquid dielectric source to be coupled in fluid communication with the liquid chamber.
A related method for heating a hydrocarbon resource in a subterranean formation having a wellbore extending therein is also provided. The method includes coupling a transmission line to an RF antenna, and coupling a balun (such as the one described briefly above) to the transmission line adjacent the RF antenna. The method further includes positioning the RF antenna, balun, and transmission line within the wellbore, filling the liquid chamber with a dielectric liquid, and supplying an RF signal to the transmission line from an RF source.
BRIEF DESCRIPTION OF THE DRAWINGS
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FIG. 1 is a schematic block diagram of an apparatus for heating a hydrocarbon resource in a subterranean formation in accordance with the present invention.
FIG. 2 is a schematic cross-sectional diagram showing the transmission line, liquid dielectric balun, and liquid tuning chambers from the apparatus of FIG. 1.
FIG. 3 is a cross-sectional perspective view of an embodiment of the balun from the apparatus of FIG. 1.
FIG. 4 is a graph of choking reactance and resonant frequency for the balun of FIG. 4 for different fluid levels.
FIG. 5 is a schematic cross-sectional view of an embodiment of the lower end of the balun of FIG. 2, showing an approach for adding/removing fluids and/or gasses therefrom.
FIG. 6 is a schematic circuit representation of the balun of FIG. 2 which also includes a second balun.
FIG. 7 is a perspective view of a transmission line segment coupler for use with the apparatus of FIG. 1.
FIG. 8 is an end view of the transmission line segment coupler of FIG. 7.
FIG. 9 is a cross-sectional view of the transmission line segment coupler of FIG. 7.
FIG. 10 is a cross-sectional view of the inner conductor transmission line segment coupler of FIG. 7.
FIGS. 11 and 12 are fully exploded and partially exploded views of the transmission line segment coupler of FIG. 7, respectively.
FIG. 13 is a schematic block diagram of an exemplary fluid source configuration for the apparatus of FIG. 1.
FIGS. 14-16 are flow diagrams illustrating method aspects associated with the apparatus of FIG. 1.
FIG. 17 is a Smith chart illustrating operating characteristics of various example liquid tuning chamber configurations of the apparatus of FIG. 1.